1. Field of the Invention
This invention relates generally to a method for preparing purified aromatic carboxylic acids and a means for effecting such purification. More particularly, this invention relates to a method for preparing purified aromatic carboxylic acids such as purified terephthalic acid, purified isophthalic acid and purified naphthalenedicarboxylic acid wherein the purified aromatic carboxylic has a reduced level of particulate contaminates.
2. Discussion of the Prior Art
Polymer grade aromatic carboxylic acids such as "purified" terephthalic acid or isophthalic acid are the starting materials for polyethylene terephthalates and isophthalates, respectively, which are the principal polymers employed in the manufacture of polyester fibers, polyester films, and resins for bottles and like containers. Similarly, polymer grade or "purified" naphthalene dicarboxylic acids, especially 2,6-naphthalene dicarboxylic acid, are the starting materials for polyethylene naphthalates, which can also be employed in the manufacture of fibers, films and resins. A purified terephthalic acid, isophthalic acid or naphthalene dicarboxylic acid can be derived from a relatively less pure, technical grade or "crude" terephthalic acid, isophthalic acid or "crude" naphthalene dicarboxylic acid, respectively, by purification of the crude acid utilizing hydrogen and a noble metal catalyst, as described for terephthalic acid in U.S. Pat. No. 3,584,039 to Meyer. In the purification process, the impure terephthalic acid, isophthalic acid or naphthalene dicarboxylic acid is dissolved in water or other suitable solvent or solvent mixture at an elevated temperature, and the resulting solution is hydrogenated, preferably in the presence of a hydrogenation catalyst, which conventionally is palladium on a carbon support, as described in Pohlmann, U.S. Pat. No. 3,726,915. This hydrogenation step converts the various color bodies present in the relatively impure phthalic acid or naphthalene dicarboxylic acid to colorless products. Another related purification-by-hydrogenation process for aromatic polycarboxylic acids produced by liquid phase catalyst oxidation of polyalkyl aromatic hydrocarbons is described in Stech et al., U.S. Pat. No. 4,405,809.
Carbon is conventionally used as the support material for the noble metal in the catalyst employed in the aforesaid purification method. A common disadvantage of the use of a carbon support is that carbon fines are often generated during commercial operations. The generation of such fines can be minimized but generally cannot be completely avoided. During the subsequent polymerization process to make polyester materials, such particulates introduced with the particular purified acid, for example, terephthalic acid, isophthalic acid or 2,6-naphthalene dicarboxylic acid, can plug filters and thereby cause interruptions in the process. Other such particulates that bypass the filter may be incorporated into the resulting polyester fiber or film and cause fiber breakage or film distortion.
In a typical reactor use to purify aromatic carboxylic acids, screens or other retaining means are employed for retaining the catalyst in the reactor vessel yet providing for the passage of the liquid reaction mixture containing the dissolved aromatic carboxylic acid being purified. Most of these purification reactors are arranged in a vertical fashion with the feed stream comprising aromatic carboxylic acid and solvent entering an upper part of the reactor. This solution passes through the reactor catalyst bed and exits at the bottom of the reactor. In this configuration, the catalyst particles are forced against the screen or other retaining means. Even if used in a horizontal position or where the flow is upward in a vertically arranged reactor, the flow of the reaction mixture through the reactor forces the catalyst bed against the retaining screen. The hard screens pressing against the soft carbon catalyst, which are usually irregularly shaped, cause an abrasion or attrition of the catalyst particles, and the fine carbon particles that break lose from the catalyst particles readily pass through the screen or other retaining means and contaminate the purified aromatic carboxylic acid. While it is possible to use catalyst support formulations that are more resistant to attrition, such as the catalysts described in U.S. patent application Ser. No. 08/029,037, filed on Mar. 10, 1993, it would be desirable to continue to use the catalyst based on carbon supports and having one or more hydrogenation catalyst compounds, such as Group VIII catalyst metals, deposited thereon. Because of the highly corrosive conditions under which the aforesaid purification is performed, it has proven difficult to develop suitable noncarbon catalyst supports for use in the purification catalyst. Thus, it would be highly desirable to have a method for the purification of aromatic carboxylic acid that utilizes carbon carrier materials for the noble metal catalyst yet does not introduce as much carbon particles in the purified aromatic carboxylic acid. The present invention provides such a method for the preparation of purified aromatic carboxylic acids such as purified terephthalic acid, purified isophthalic acid and purified naphthalenedicarboxylic acid, as well as a means for effecting such purification.
M. Bankmann, R. Brand, B. H. Engler and J. Ohmer, "Forming of High Surface Area TiO.sub.2 to Catalyst Supports," Catalysis Today, Vol. 14, pages 225-242 (1992), contains an extensive discussion of the use of titanium dioxide having a high surface area as a catalyst support.
Schroeder et al., U.S. Pat. No. 4,743,577, discloses that the use of catalysts containing palladium finely dispersed on carbon in the aforesaid purification of terephthalic acid derived from the oxidation of p-xylene results in contamination of the resulting purified terephthalic acid with fines produced by abrasion of the carbon granulates due to their relatively low crush strength and poor abrasion resistance. This patent discloses that reduced fines contamination results from the use instead of a catalyst containing a thin layer of palladium, nickel, rhodium, platinum, copper, rhuthenium and cobalt on a porous sintered support of metallic titanium, zirconium, tungsten, chromium, nickel and alloys incorporating one or more of these metals. The surface area of palladium-plated supports of titanium, inconel and nickel are disclosed as 0.22, 0.55 and 1.21 square meters per gram, respectively, which are very significantly smaller than specific surface area of a palladium on active carbon catalyst.
Schroeder et al., U.S. Pat. No. 4,629,715, discloses that aqueous solution of crude terephthalic acid can be purified to relatively low 4-carboxy-benzaldehyde levels under hydrogenation conditions by using a layered catalyst bed wherein the first layer is constituted by palladium supported on an active carbon carrier and the second layer is constituted by rhodium supported on an active carbon carrier.
Schroeder et al., U.S. Pat. No. 4,892,972, discloses a method for the purification of a crude terephthalic acid by passing on aqueous solution of crude terephthalic acid in the presence of hydrogen through a first catalyst layer containing a Group VIII metal supported on a carbon carder wherein the Group VIII metal has a single electron in its outermost orbital in the ground state, and a second layer containing palladium supported on an active carbon carrier.
Sikkenga et al., pending U.S. patent application Ser. No. 07/900,593, filed Jun. 18, 1992, discloses the preparation of an aromatic carboxylic acid by the liquid phase catalyzed oxidation of an alkyl-substituted aromatic compound such as o-, m-, or p-xylene or 2,6-dimethylnaphthalene. The application further discloses on page 11, lines 23-31, that the resulting aromatic carboxylic acids can be purified by hydrogenation thereof in the presence of a catalyst comprising one or more Group VIII metals deposited on a support such as alumina, titania or carbon.
Holzhauer et al., pending U.S. patent application Ser. No. 07/900,637, filed Jun. 18, 1992, discloses on page 27, lines 1-12, a method for purifying 2,6-naphthalene dicarboxylic acid by treating it with hydrogen in the presence of a hydrogenation catalyst containing one or more of platinum, palladium, rhodium, ruthenium, osmium and iridium supported on alumina, silica-alumina, silica, titania, clays and zirconia.